1. Field of Invention
The present invention relates to an internal combustion engine system having a combustion chamber into which inert gas is introduced so as to cause combustion.
2. Description of Related Art
Conventionally, in an internal combustion engine, for example, in a diesel engine, in order to restrict the generation of NOx, an engine exhaust passage and an engine intake passage are connected by an exhaust gas recirculation (hereinafter, referred to as EGR) passage so as to recirculate exhaust gas, that is, EGR gas into the engine intake passage via the EGR passage. In this case, the EGR gas has a relatively high specific heat and is able to absorb a large amount of heat. Hence, as the amount of EGR gas is increased, that is, as the EGR rate (the amount of EGR gas/(the amount of EGR gas+the amount of intake air)) is increased, the combustion temperature in a combustion chamber decreases. If the combustion temperature decreases, the generation amount of NOx is reduced. Thus, the larger the EGR rate becomes, the smaller the generation amount of NOx becomes.
As described hitherto, it has been conventionally known that the generation amount of NOx is reduced by increasing the EGR rate. However, in the process of increasing the EGR rate, when the EGR rate exceeds a certain threshold value, the generation amount of soot, namely smoke, starts increasing abruptly. In this respect, it has been conventionally considered that if the EGR rate is increased beyond this certain threshold value, the amount of smoke increases endlessly. In other words, the EGR rate at which the amount of smoke starts increasing abruptly has been considered to be a maximum allowable value of the EGR rate.
Accordingly, the EGR rate conventionally has been set below the maximum allowable value. Although the maximum allowable value of the EGR rate differs greatly depending on a type of the engine or fuel, it is approximately 30% to 50%. Therefore, in the diesel engine, the EGR rate is at most set to 30% to 50%.
As described above, it has been conventionally considered that the EGR rate has a maximum allowable value. Thus, the EGR rate is set so as to reduce a generation amount of NOx and smoke to the maximum possible extent, by ensuring that the EGR rate does not exceed the maximum allowable value. However, even if the EGR rate has been thus determined, the generation amount of NOx and smoke cannot be reduced limitlessly. In fact, the generation of a considerable amount of NOx and smoke is inevitable.